Solid state electrochromic mirror

ABSTRACT

Electro-optical device particularly useful in the modulation of reflected light typically comprising an electrochromic device containing a reflecting layer electrode in sandwiched arrangement, said electrochromic device being a sandwich arrangement of a transparent electrode, a reflecting layer electrode, a film of a transition metal compound and a solid insulating film disposed between the electrodes. The electrochromic device exhibits coloration and bleaching thereof at ambient temperature by control of the polarity of an applied electric field, whereby light reaching the reflecting solid layer electrode is modulated in intensity, thus modulating, in turn, the reflected light.

United State: n11 3,712,710 Castellion et al. 1 Jan. 23, 1973 s41 SOLIDSTATE ELECTROCHROMIC 3,521,941 7 1970 Deb et al. .350/160 MIRROR [75]Inventors: George Augustus Castellion, Stamford; Donald Paul Spitzer,Riverside both of Conn.

[73] Assignee: American Cyanamid Company,

Stamford, Conn.

[22] Filed: Dec. 21,1970

[2]] Appl. No.: 99,909

[52] U.S. Cl .350/160 [51] Int. Cl ...G02f1/36 [58] Field of Search..350/160 [56] References Cited UNITED STATES PATENTS 3,578,843 5/l97lCastellion ..350/l60 Primary Examiner-William L. Sikes Attorney-CharlesJ. Fickey [57] ABSTRACT Electra-optical device particularly useful inthe modulation of reflected light typically comprising an electrochromicdevice containing a reflecting layer electrode in sandwichedarrangement, said electrochromic device being a sandwich arrangement ofa transparent electrode, a reflecting layer electrode, a film of atransition metal compound and a solid insulating film disposed betweenthe electrodes. The electrochromic device exhibits coloration andbleaching thereof at ambient temperature by control of the polarity ofan applied electric field, whereby light reaching the reflecting solidlayer electrode is modulated in intensity, thus modulating, in turn, thereflected light.

8 Claims, 3 Drawing Figures PATENTED 7 I 3,712,710

flfi. E /15. 5

INVENTORS, GEORGE AUGUSTUS CASTELL/O/V DON/1L0 PAUL SP/ 725/? ATTORNEY lSOLID STATE ELECTROCHROMIC MIRROR BACKGROUN D OF THE lNVENTlON ln ourcopending, application, Ser. No. 622,822, filed Feb. 7, 1967 and nowabandoned, there are described electro-optical devices exhibiting aphenomenon known as persistent electrochromism. This term denotes theproperty of a material, whereby its electromagnetic radiation absorptioncharacteristic is altered at ambient temperature under the influence ofan electric field. Such materials as for example, tungsten oxide andmolybdenum oxide may exhibit little or no absorption of visiblewavelengths in the absence of an electric field and, therefore, aretransparent. When subjected to an electric field, however, theyeffectively absorb in the red end of the spectrum, turning blue incolor. If the electrodes and the electrochromic layer are formed on thesurface of a transparent substrate, such as glass, or clear acrylicresin, the light transmitting characteristics of the combination can bevaried by controlling the electric field produced across theelectrochromic layer. Thus, if the sandwich" of electrodes andelectrochromic material on the substrate originally is clear,application of a voltage across the electrodes to establish an electricfield of the proper polarity changes the light absorption characteristicof the electrochromic material, turning it darker for example, thusdecreasing the light transmitting ability of the entire assembly.

As further described in our copending U.S. application, Ser. No.773,690, filed Sept. 25, 1968 and now abandoned, when an additionallayer of solid state material such as silicon oxide or calcium fluoridecharacterized as a current carrier permeable insulator is placed betweenone of the electrodes and the electrochromic material it not onlypermits the absorption characteristic of the electrochromic material tochange rapidly under the influence of an electric field of a givenpolarity but it also renders the electrochromic layer sensitive to afield of the opposite polarity to return it positively to the absorptionrate it occupied prior to the initial application of the field. Thisconcept was applied in the control of light reflected from a mirror.

While the mirror device of the foregoing disclosure functionedsatisfactorily it had a relatively limited service life since thecurrent carrier permeable insulator was not chemically andelectrochemically matched to the counterelectrode in contact therewith.If such service life could be extended for an indefinite period of timesuch a solution would fulfill a long felt need in the art.

The device of the present disclosure overcomes the deficiency of theprior art device by employing a 3 alumina insulating material chemicallyand electrochemically compatible with an alkali metal counterelectrodeincorporating a mirror surface as a integral part of thecounterelectrode in a sealed system.

Unexpectedly, the result of this combination of a mirror electrodeelement in direct contact with a compatible insulator material extendsexpected service life indefinitely.

SUMMARY OF THE INVENTION It is a principal object of this invention toprovide a solid state reflecting device for modulating reflected light.

Another object of the present invention is to provide an improved formof electro-optical light modulating device wherein a current carrierpermeable insulator can be placed directly in contact with a reflectingcounterelectrode layer so as to provide prolonged service life in asealed unit for rapid and even coloration at low applied potential.

A further object of the invention is to provide a solid state lightreflecting device having infinitely variable light modulation ability.

Briefly stated, the device of the present invention comprises insandwich arrangement, a transparent electrode, a persistentelectrochromic layer, a current carrier permeable insulator and areflecting electrode additionally serving as a mirror layer. inoperation, when the device of the present invention is placed in anelectric circuit with the transparent electrode negative and thereflecting electrode positive, the electrochromic layer will becomecolored. Thus, the amount of light reaching the reflecting surface fromthe side of the transparent electrode and being reflected back will bereduced. Reversing the circuit polarity will cause the electrochromiclayer to bleach to the colorless state, thus increasing the reflectedlight from the reflecting surface to its original intensity.

The foregoing and other features, objects and advantages of the presentinvention will become more apparent from the following detaileddescription thereof taken in conjunction with the accompanying drawingsin which:

HO. 1 is an illustration, partly in cross-section, of an electro-opticaldevice of the present invention, type described and claimed in theforegoing earlier applications; and

FIGS. 2 and 3 are diagrammatic illustrations of the inventive device inthe modulation of reflected radiation.

In the description herein the device of the present invention isdescribed in terms of its effect on visible light transmissioncharacteristics, i.e., the variation in the wavelength absorption of theelectrochromic material within the visible region of the spectrum. itwill, of course, be recognized that the phenomenon exhibited by the typeof materials to be described is not limited to the visible spectrum butmay extended into the invisible regions.

ELECTROCHROMIC MATERIALS As a critical element of the device definedhereinafter, there is employed a persistent electrochromic material." Itis defined as a material responsive to the application of an electricfield of a given polarity to change from a first persistent state inwhich it is essentially non-absorptive of electromagnetic radiation in agiven wavelength region to a second persistent state in which it isabsorptive of electromagnetic radiation in the given wavelength region.Once in said latter state, said persistent electrochromic material isresponsive to the application of an electric field of the oppositepolarity to return to its original state. Certain of such materials canalso be responsive to a short circuiting condition, in the absence of anelectric field, so as to return to the initial state.

By persistent" is further meant the ability of the material to remain inthe absorptive state to which it is changed, after removal of theelectric field, as distinguished from a substantially instantaneousreversion to the initial state, as in the case of the well-known Franz-Keldysh effect.

The materials which form the electrochromic materials of the device ingeneral are electrical insulators or semiconductors. Thus are excludedthose metals, metal alloys, and other metal-containing compounds whichare relatively good electrical conductors.

While not wholly understood, it appears that the materials contain innon-stoichiometric proportions at least two different elements, saidelements being present as ions of opposite polarity. This conditionproduces lattice defects as distingusihed from mere physicaldisplacement of crystal symmetry, although the condition may also resultin or be evidenced by such. Lattice vacancies are particular instancesof lattice defects as, for example, an oxygen vacancy in 'a metal oxidecrystal.

Two classes of electrochromic materials may be distinguished whichsatisfy the foregoing conditions and are therefore useful in the deviceof the present invention. The first and preferred class (1) comprisesmaterials disclosed in the above-mentioned prior patent application.These materials exhibit persistent electrochromism over a widetemperature range including ambient temperature and in some instanceshigh temperatures, e.g., above about 125C. or low temperatures, e.g.,below about -50C. By ambient temperature" is meant temperatures normallyencountered in the fields of use of the devices such as describedhereinafter, e.g., 50C. to 125C.

The second class (ll), comprises materials which exhibit persistentelectrochromism only at relatively high (nonambient) temperature, e.g.,above about 125C. Examples of these materials are gross crystals orcrystalline layers or films of alkali halides such as NaCl, RbCl, KCl,LiF, NaBr, KBr, Kl, RbBr, and the like, as described in British Pat. No.845,053 and corresponding West German Pat. No. 1,036,388. Combinationsof class (I) and class (ll) materials may also be employed.

The class (1) materials are further characterized as inorganicsubstances which are solid under the conditions of use, whether as pureelements, alloys, or chemical compounds, containing at least one elementof the Periodic System which can exist in more than one oxidation statein addition to zero. The term oxidation state as employed herein isdefined in lnorganic Chemistry," T. Moeller, John Wiley and Sons, Inc.,New York, 1952. These include materials containing a transition metalelement (including Lanthanide and Actinide series elements); materialscontaining non-alkali metal elements such as copper, tin and barium; andmaterials containing an alkali metal element with a variable oxidationstate element. Preferred materials of this class are films of transitionmetal compounds in which the transition metal may exist in any oxidationstate from +2 to +8. Examples of these are: transition metal oxides,transition metal sulfides, transition metal oxysulfides, transitionmetal halides, selenides, tellurides, chromates, molybdates, tungstates,vanadates, niobates, tantalates, titanates, stannates, and the like.Particularly preferred are films of metal stannates, oxides and sulfidesof the metals of Groups lVB, VB and WE of the Periodic System, andLanthanide series metal oxides and sulfides. Examples of such are copperstannate, tungsten oxide, molybdentfm oxide, cobalt tungstate, metalmolybdates, metal titanates, metal niobates, and the like.

The class (I) electrochromic materials are distinguished from priorknown organic or inorganic materials which exhibit coloration in anelectric field as a result of the Franz-Keldysh effect or the effectPlatt describes as electrochromism." As to Platt, see J. Chem. Phys. 34,862-3 (1961). In the latter cases, coloration results from the shiftingof an existing absorption band or spectral line by the electric field;whereas in the present invention, upon coloration, an absorption band iscreated where none existed before, or removed upon bleachinG.

An important advantage of devices of the invention containing a class(I) persistent electrochromic material is operability at ambienttemperature. So far as is known, this is the first instance ofelectrochromic behavior at temperatures of practical application. Theinvention, therefore, permits numerous practical applications to whichprior art electro-optical devices are not susceptible as will be evidentfrom the ensuing description.

When the persistent electrochromic materials are employed as films,thickness desirably will be in the range of'from about 0. l-l00 microns.However, since a small potential will provide an enormous field strengthacross very thin films, the latter, i.e., 0.1-10 microns, are preferredover thicker ones. Optimum thickness will also be determined by thenature of the particular compounds being laid down as films and by thefilmforming method since the particular compound and film-forming methodmay place physical (e.g., nonuniform film surface) and economiclimitations on manufacture of the devices.

The films may be self-supporting, depending on thickness and filmmaterial, or may be laid down on any substrate which, relative to thefilm, is electrically nonconducting. Suitable substrate materialsinclude glass, wood, paper, plastics, plaster, and the like, includingtransparent, translucent, opaque or other optical quality materials.

The preferred electrochromic material for use with the insulating layeris a class (1) material as defined above. However, the performance ofclass (ll) electrochromic materials is also improved since theelectrochromic material is made polarity sensitive thereby, that is,responsive to a field of one polarity but not to both at the same timeas in the prior art device of British Pat. No. 845,053.

INSULATING LAYER The insulating layer may be defined as a currentcarrier permeable insulator" and as used herein is intended to denoteany material of electrical resistivity sufficient to provide continuouseffective insulation against normal electrical conduction betweenopposed surfaces of the electrodes. Numerous well-known solid statematerials are suitable for use as current carrier permeable insulatorsin this invention. These include inorganic compounds of groups IA, HA,and [HA elements such as alkali oxide, alkaline earth oxides andalumina.

The insulating layer of the present disclosure generally comprises mixedalkali and alkaline earth metal oxides such as sodium oxide, potassiumoxide and magnesium oxide in combination with aluminum oxide. As tothese materials see 1. Inorganic and Nuclear Chem 29 2453-2475 (1969).Beta alumina, Na O llAl O is a preferred insulating material because ofits ease of handling and commercial availability. Suitable currentcarrier permeable insulators are additionally characterized as colorlesssolid ionic conductors having bulk ionic conductivities greater than/ohm/cm which permit the passage of only the ion wanted. Furthermore,such materials must be chemically stable and electrochemicallycompatible with the materials of the mirror electrode.

Preferably the insulator is a film of at least 1.0 micron thickness, forexample, in the range of about 1.0 to 20.0 microns.

The mechanism by which the current carrier permeable insulator improvesthe performance of the persistent electrochromic material can beunderstood as a selective introduction of charge carriers (i.e.,electrons, holes, positive or negative ions) suitable for the subsequentproduction of persistent coloration in the electrochromic material. Thecurrent carrier permeable insulator thereby renders the electrochromicmaterial polarity sensitive, with the result that application of avoltage of polarity opposite that which produces coloration will resultin bleaching without simultaneous recoloration.

This general mechanism may be viewed more particularly as two cases ortheories, electronic and ionic. Each case explains certain observationsnot adequately explained by the other case, and it is not altogetherimplausible that the mechanisms, may operate simultaneously althoughindependently.

In a first or electronic case, the current carrier permeable insulatorfunctions by non-classical transposition (tunneling) of electrons orholes through the energy barrier junction between the insulator and thepersistent electrochromic material. An equivalent characterization ofsuch insulator materials in this view is that they exhibit an energy gapbetween their valence and conduction bands of width sufflcient at thetemperature of use to impede normal electrical conduction through thematerial of the insulator but nevertheless, because of their thinness,permit quantum mechanical tunneling of current carriers, i.e., electronsor holes, The current carriers which are injected by the tunnelingprocess through the insulator into the persistent electrochromicmaterial possess sufficient energy to become trapped in the energy levelsites which produce the color centers observed as the coloration of thepersistent electrochromic material. in order to maintain approximatecharge neutrality in the persistent electrochromic layer, carriers ofsign opposite to that of carriers which tunnel through the insulatinglayer must enter from the electrode opposite the electrode adjacent theinsulating layer. During bleaching, either by shortcircuiting or byimposition of a voltage opposite that of the voltage which producescoloration, the charge carriers are removed or permitted to recombinethrough the external circuit, emptying the carriers from their traps andthus restoring the color centers to their original colorless condition.Coloration cannot occur under the condition of reverse voltage becausethe current carrier permeable insulator is not adjacent the electrode ofpolarity suitable for the tunneling and injection phenomenon.

Alternatively, as a second case, the current carrier permeable insulatorcan serve to block entirely the passage of an electronic current (i.e.,electrons or holes) but permit the transfer through it of ions. in suchcase, the insulator serves to facilitate the production of color centersin the persistent electrochromic layer by providing a large electricfield gradient through which ions may move rapidly, even at ambienttemperature, to be removed or added to the persistent electrochromicmaterial. In this situation, the insulator layer can also serve as atemporary or permanent repository for ions removed from theelectrochromic layer.

Whether or not these theories are ultimately proven to govern in thepresent invention, the devices described herein achieve the colorationand bleaching capabilities indicated.

ELECTRODES A large variety of materials exhibiting electricalconductivity and light transmission and reflection characteristics maybeused for electrodes. The same material may be used for both electrodesor each electrode may be of a different material, or mixture of alloysof different material. Typical electrode materials are the metals, e.g.,sodium, potassium, lithium and rubidium and conducting non-metals suchas suitably doped tin or indium oxide and the like. One of theelectrodes should be of an optical quality effective for transmission ofthe electrochromic change, if in the visible or for instrumentallysensing the change, if not in the visible range. The other electrodeshould be of reflecting quality effective as a mirror surface forvisible light. Additionally, this mirror electrode should be chemicallyand electrochemically matched with the current carrying semipermeableinsulating layer.

The mirror films are self supporting, depending on thickness and filmmaterial or are laid down on any substrate which relative to the film iselectrically nonconducting and chemically inert. Suitable substratematerials include glass, plastics and the like, including transparent,translucent, opaque or other optical quality materials.

Turning now to the drawings, FIG. 1 illustrates a view in cross sectionof a device 20 in accordance with the invention. On a substrate 22 suchas glass or other transparent material, are successively depositedlayers of a conductive and reflecting material 24, an insulatingmaterial 27, a persistent electrochromic material 26, and a secondconductive material 28. The conductive material 28 is of optical qualityeffective for passing light to reflecting electrode layer 24. Thesubstrate 22 and the conductive layer 24 may conveniently be provided asa unit by a front surfaced sodium mirror. The reflecting electrode 24may be any known material which will reflect light such as sodium andpotassium metal film and is compatible to the insulating material. Thelayers 27 and 26 may then be deposited on the sodium mirror layer byknown vacuum deposition or sputtering techniques. The persistentelectrochromic material may be tungsten oxide or molybdenum oxide. Theouter electrode 28 may conveniently be provided as a unit by so-calledNESA" glass, a commercially available product having a transparentcoating of conductive tin oxide on one surface of a glass sheet. Asource of DC potential 30 is coupled between the conductive films withits positive terminal on the sodium layer and its negative terminal onthe tin oxide outer layer.

The negative and positive electrodes need only be in electrical contactwith the film. Any type and arrangement of electrodes and film effectiveto impose an electric field on the film when the electrodes areconnected to a voltage source, will be suitable. Thus, the electrodesmay be spaced conducting strips deposited on or inbedded in the film, orpreferably they may be conducting layers between which the film isinserted.

The device functions effectively in a reversible manner. For thispurpose, the battery 30 is coupled to the electrodes 24 and 28 through areversing switch indicated generally at 36. As shown, with the switcharm in the position to produce coloration, the positive terminal of thesource is connected to the inner or sodium electrode while the negativeterminal is connected to the tin oxide layer on the glass substrate.

Once complete coloration is induced, which in a typical case is a matterof seconds, the switch 36 may be opened, disconnecting the battery fromthe device entirely, and the device will remain in its darkened statewithout further application of power.

To bleach or erase a previously darkened surface, the switch arm isthrown to the bleach" contacts, across which is connected apotentiometer 37. As shown, the potentiometer contact or slider ismovable from a point at which the electrodes 24 and 28 are shortcircuited to a point at which full battery voltage, of polarity oppositeto the coloration condition, is applied between them. Any number ofreverse voltage values may be obtained between the two extremes.

in the position illustrated in the drawing, a bleach" voltage of a valueless than battery voltage is applied across the electrodes, setting up acorresponding electric field. Under the influence of this field, thedevice returns to its initial uncolored state. The rapidity with whichthe bleaching occurs is determined by the magnitude of the voltage; thehigher the voltage; the faster the bleaching process is completed. Atthe higher bleaching voltages, it has been found that the bleachingprocess is even faster than the coloring operation. Once the bleachingis completed, no further coloration is observed with this polarity andthe switch may be opened to disconnect the battery from the device andminimize power drain.

it has also been found that, notwithstanding the absence of an electricfield, when the potentiometer is in its short circuiting position,certain of the persistent electrochromic materials nevertheless willreturn completely and positively to the initial state. The rate at whichthe bleaching occurs, however, is somewhat slower than when the materialis subjected to an electric field.

Thus, the device of FIG. 1 functions as a self-contained modulator forreflected light. As shown in FIG. 2, a light ray 40 is reflected as asubstantially full intensity ray 41 when the device 20 is in a bleachedstate. When the device is colored, as shown in FIG. 3, the amount oflight from ray 40 passing through the colored electrochromic layer 26 tothe reflecting layer 24 and from the reflecting layer is less due toabsorption. Thus the reflected ray 41A, is lower in intensity. Thedifference in intensity may be varied as desired by controlling thedensity of coloration of electrochromic layer 26. The coloration is afunction of the time that the current is applied, up to a certainmaximum coloration. Thus if the current is applied for any time intervalless than that required to obtain maximum coloration, a lesser amount ofcoloration will be obtained which will absorb less light, giving morereflected light. The amount of reflected light may thus be varied fromthe total, to any amount down to the minimum allowed.

Moreover, when the coloration current is cut off, the state ofcoloration reached at that point persists and does not require constantapplication of current to be maintained.

As will be apparent from the specific examples to be described below,many combinations of persistent electrochromic materials, insulatingmaterials and electrode materials may be employed in accordance with thepresent invention.

EXAMPLE 1 A film of molybdenum oxide, about 1.0 micron in thickness, isthermally evaporated by conventional means at a pressure of 10- Torr.from an electrically heated tantalum boat onto the tin oxide coated sideof NESA" glass, the tin oxide on the glass forming the transparentelectrode. A film of Na O "Al 20;, (about 3 microns thick) forming aninsulating material, is then deposited by sputtering onto the molybdenumoxide layer. Finally, a layer of sodium (about 0.5 micron thick)effectively reflecting is deposited over the Na o llAl O insulatinglayer to form the second electrode and mirror surface of the layeredstructure or sand- Wich.

During the several stages of evaporation and sputtering appropriatemasking is effected to expose a portion of the tin oxide layer forattachment of the conductor, and also to extend the sodium layer so thata portion of it is directly on an uncoated portion of the glasssubstrate, minimizing the danger of shorting through to the tin oxidelayer when the conductor is attached to the sodium electrode.

When an electric field of from 1 to 3 volts is applied across theforegoing sandwich structure with the sodium layer as the positiveelectrode and the tin oxide as the negative electrode, the molybdenumoxide film, normally colorless, is colored blue uniformly over theentire surface, reducing the reflected light transmission of the deviceto about 10 percent in 30 seconds. The coloration remains substantiallypermanent when the electric field iS removed.

When an electric field of reverse polarity is applied, i.e., positivepotential on the tin oxide layer and negative potential on the sodiumlayer, the coloration fades uniformly and completely to restore theinitial reflected light transmission of the sandwich. This occurssomewhat faster than the coloration, taking about 6 to l5 seconds, butcan be varied by changing the value of potential.

EXAMPLE 2 The device is fabricated as described in connection withExample 1 except that a potassium oxide B-alumina insulating layer andpotassium mirror is substituted for the Na,0'llAl,O, insulating layersodium mirror system. Application of 2 to 3 volts between theelectrodes, with the sodium electrode positive, reduces the reflectedlight transmission of the device to 4 percent in about 2 minutes.Reversal of the polarity for approximately 15 seconds restores the fulllight transmission capability. It has been found that with themolybdenum oxide film, bleaching also occurs, but more slowly, when theelectrodes are short circuited. A very gradual, i.e., over a span ofseveral hours, bleaching occurs also with the field removed and theelectrodes open circuited.

EXAMPLES 3-12 Table l below illustrates other combinations of insulationmaterials and mirror electrode materials which when supported as filmssubstantially as described in Examples 1 and 2 exhibit the reflectedradiation transmission characteristics of the invention.

TABLE I Example Insulation Material Mirror Electrode 3 Na, llAl,O,potassium 4 2Na, 0 "M 0, sodium 5 K, O llAl, 0, sodium 6 2K, 0 llAl, O,potassium 7 Na, 0 llAl, O sodium-potassium alloy 8 Na, 0 Al, 0;,sodium-mercury amalgam l0 Na, 0 MgO SM, 0; sodium-mercury amalgam l l K,O MgO SM, 0, potassium-mercury amalgam 12 Na, 0 MgO SA], 0,sodium-potassium alloy Other combinations of the materials discussedabove may be employed to vary the final characteristics of the overalldevice, i.e., the percentage change in light transmission capability,the voltage required to establish the requisite field strength, the timefor the change to occur, etc. The depth of coloration is also dependentupon the thickness of the persistent electrochromic layer. In theory, itwould seem that the thicker the layer, the more color centers would beformed upon application of the electric field and therefore deepercoloration could be expected. However, since thin layers could beexpected to color more quickly in some cases, the relationship betweenthickness of the layers and depth of color is not simple.

The inventive device can be useful in many ways. It can be used as partof an optical system involving reflective elements where close controlof light intensity is desired without modifying its other properties.Thus no diaphragams or other separate light modulating elements would benecessary. Moreover, the optics may be simpler since the light rays arenot altered except in intensity.

While the device has been illustrated as having a flat reflectingsurface, it will be obvious that the reflecting surface may take anydesired configuration such as a spherical or parabolic surface, forexample.

The device is particularly suitable as a rear view mirror in motorvehicles for night driving. It is possible by the use of the device toreduce the intensity of reflected light from headlights of a followingvehicle to a desired degree by merely coloring the electrochromic layer.This can be done by mere switching and is thus quick and effective.

While certain specific embodiments and preferred modes of practice ofthe invention have been described, this is solely for illustration, andit will be obvious that various changes and modifications may be madewithout departing from the spirit of the disclosure or the scope of theappended claims.

We claim:

1. A radiation reflective device having an electric field responsiveradiation transmitting characteristic comprising:

a. a pair of conductive electrodes between which an electric field isestablished;

b. an insulating material consisting essentially of beta-alumina incontact with one of said electrodes; and

c. a persistent electrochromic material in contact with said insulatingmaterial and disposed between said electrodes wherein said electrode incontact with the insulating layer is a light reflecting layer ofevaporated sodium and the other of said electrodes is substantiallytransparent.

2. A radiation reflective device having an electric field responsiveradiation transmitting characteristic comprising:

a. a pair of conductive electrodes between which an electric field isestablished;

b. a current carrier permeable insulator selected from the groupconsisting of inorganic compounds ofGroups lA, [IA and [HA and incontact with one of said electrodes,

c. a persistent electrochromic material in contact with said insulatingmaterial and disposed between said electrodes, wherein said electrode incontact with the insulating layer is a light reflecting layer ofevaporated sodium and the other of said electrodes is substantiallytransparent.

3. The device of claim 2 wherein said persistent electrochromic materialis tungsten oxide.

4. The device of claim 2 wherein said persistent electrochromic materialis molybdenum oxide.

5. A radiation reflective device having an electric field responsiveradiation transmitting characteristic comprising, in sandwicharrangement layers in contiguous contact in the following order:

a. an electrode comprising as a reflective material, an

alkali metal or doped conducting metal oxide,

b. a current carrier permeable insulator selected from the groupconsisting of colorless solid ionic conductors having bulk ionicconductivities greater than l0/ohm/cm, said reflecting electrode andinsulator having chemical and electrochemical compatibility,

c. a persistent electrochromic material,

d. a transparent electrode.

6. A variable light transmission device as defined in claim 5comprising:

control means coupled to said device for selectively applying acrosssaid electrodes a potential of one polarity, a potential of oppositepolarity and an effective short circuit.

7. The device of claim 6 wherein at least one of the selectively appliedpotentials is variable over a given range.

8. A radiation reflective device having an electric trode and insulatorhaving chemical and electrochromical compatibility,

c. a persistent electrochromic material selected from the groupconsisting of metal stannates, oxides and sulfides of metals of Groups[V B, V B and VI B of the Periodic System, and Lanthanide series metaloxides and sulfides, and, d. a transparent electrode.

t i i t

2. A radiation reflective device having an electric field responsiveradiation transmitting characteristic comprising: a. a pair ofconductive electrodes between which an electric field is established; b.a current carrier permeable insulator selected from the group consistingof inorganic compounds of Groups IA, IIA and IIIA and in contact withone of said electrodes, c. a persistent electrochromic material incontact with said insulating material and disposed between saidelectrodes, wherein said electrode in contact with the insulating layeris a light reflecting layer of evaporated sodium and the other of saidelectrodes is substantially transparent.
 3. The device of claim 2wherein said persistent electrochromic material is tungsten oxide. 4.The device of claim 2 wherein said persistent electrochromic material ismolybdenum oxide.
 5. A radiation reflective device having an electricfield responsive radiation transmitting characteristic comprising, insandwich arrangement layers in contiguous contact in the followingorder: a. an electrode comprising as a reflective material, an alkalimetal or doped conducting metal oxide, b. a current carrier permeableinsulator selected from the group consisting of colorless solid ionicconductors having bulk ionic conductivities greater than 10 6/ohm/cm,said reflecting electrode and insulator having chemical andelectrochemical compatibility, c. a persistent electrochromic material,d. a transparent electrode.
 6. A variable light transmission device asdefined in claim 5 comprising: control means coupled to said device forselectively applying across said electrodes a potential of one polarity,a potential of opposite polarity and an effective short circuit.
 7. Thedevice of claim 6 wherein at least one of the selectively appliedpotentials is variable over a given range.
 8. A radiation reflectivedevice having an electric field responsive radiation transmittingcharacteristic comprising, layers in contiguous contact in the followingsandwich arrangement: a. an electrode comprising as a reflectingmaterial, an alkali metal, b. a current carrier permeable insulatorselected from the group consisting of alkali and alkaline earth metaloxides, or mixtures thereof, in combination with aluminum oxide, saidreflecting electrode and insulator having chemical and electrochromicalcompatibility, c. a persistent electrochromic material selected from thegroup consisting of metal stannates, oxides and sulfides of metals ofGroups IV B, V B and VI B of the Periodic System, and Lanthanide seriesmetal oxides and sulfides, and, d. a transparent electrode.